Numinous (/ˈnjuːmɪnəs/) means "arousing spiritual or religious emotion; mysterious or awe-inspiring"; also "supernatural" or "appealing to the aesthetic sensibility." The term was given its present sense by the German theologian and philosopher Rudolf Otto in his influential 1917 German book The Idea of the Holy. He also used the phrase mysterium tremendum as another description for the phenomenon. Otto's concept of the numinous influenced thinkers including Carl Jung, Mircea Eliade, and C. S. Lewis. It has been applied to theology, psychology, religious studies, literary analysis, and descriptions of psychedelic experiences.

Numinous was derived in the 17th century from the Latin numen, meaning "nod" and thus, in a transferred (figurative, metaphorical) sense, "divine will, divine command, divinity or majesty." Numinous is etymologically unrelated to Immanuel Kant's noumenon, a Greek term referring to an unknowable reality underlying all things.

The word was given its present sense by the German theologian and philosopher Rudolf Otto in his influential 1917 book Das Heilige, which appeared in English as The Idea of the Holy in 1923.

Otto writes that while the concept of "the holy" is often used to convey moral perfection—and does entail this—it contains another distinct element, beyond the ethical sphere, for which he uses the term numinous. He explains "numinous" as a "non-rational, non-sensory experience or feeling whose primary and immediate object is outside the self." This mental state "presents itself as ganz Andere, wholly other, a condition absolutely sui generis and incomparable whereby the human being finds himself utterly abashed."

Otto argues that because the numinous is irreducible and sui generis it cannot be defined in terms of other concepts or experiences, and that the reader must therefore be "guided and led on by consideration and discussion of the matter through the ways of his own mind, until he reaches the point at which 'the numinous' in him perforce begins to stir... In other words, our X cannot, strictly speaking, be taught, it can only be evoked, awakened in the mind." Chapters 4 to 6 are devoted to attempting to evoke the numinous and its various aspects.

Using Latin, he describes it as a mystery (Latin: mysterium) that is at once terrifying (tremendum) and fascinating (fascinans). He writes:

The feeling of it may at times come sweeping like a gentle tide pervading the mind with a tranquil mood of deepest worship. It may pass over into a more set and lasting attitude of the soul, continuing, as it were, thrillingly vibrant and resonant, until at last it dies away and the soul resumes its "profane," non-religious mood of everyday experience. [...] It has its crude, barbaric antecedents and early manifestations, and again it may be developed into something beautiful and pure and glorious. It may become the hushed, trembling, and speechless humility of the creature in the presence of—whom or what? In the presence of that which is a Mystery inexpressible and above all creatures.

Otto's use of the term as referring to a characteristic of religious experience was influential among certain intellectuals of the subsequent generation. For example, "numinous" as understood by Otto was a frequently quoted concept in the writings of Carl Jung, and C. S. Lewis. Lewis described the numinous experience in The Problem of Pain as follows: